In a conventional liquid crystal projector, as shown in FIG. 16, light emitted by a light source 101 is reflected by a reflector 102 and converted to nearly parallel rays. The light passes through a ultraviolet and infrared cutoff filter 103 to eliminate undesired wavelengths, and passes through a condenser lens 104 to be slightly condensed, and enters into a color liquid crystal panel 105.
The color liquid crystal panel 105 has a structure as shown in FIG. 17. The panel 105 includes Thin Film Transistors (TFTs) and bus lines 106, transparency pixel electrodes 107, liquid crystal layer 108, black-matrix 109 and color filter 110 between glass plates 111 and 112. The black-matrix 109 prevents undesired light from going into the TFTs. The color filter 110 is for adding color to light entering into the liquid crystal layer 108 (or to light emitted form the liquid crystal layer 108).
Referring again to FIG. 16, a polarizing plate 113 is positioned on the incident side (the side nearest to the light source 101) of the color liquid crystal panel 105. The polarizing plate 113 may be attached to the condenser lens 104, as shown in FIG. 16. Another polarizing plate 114 placed on the emanating side (a side near to the projection lens 116) of the color liquid crystal panel 105 is attached directly to the liquid crystal panel 105. To cool the color liquid crystal panel 105 and polarizing plate 114 on the emanating side of the liquid crystal panel 105, a liquid cooler 115 is attached to the polarizing plate 114 on the emanating side of the liquid crystal panel 105. The liquid cooler 115 includes a heat sink 115a and a container 115b in which coolant 115c is included. An image formed on the liquid crystal panel 105 is projected on the screen (not shown) by a projector lens system 116.
In this conventional single plate liquid crystal projector, the increase in temperature in some parts caused by the absorbed light is higher than that of another type of projector having three liquid crystal panels corresponding to three different colors. This is because all the light energy irradiated from the light source 101 is applied to a single color liquid crystal panel 105. For example, when a combination of light source including a 150 watt metal halide lamp and a 3 inch color liquid panel is used, it is observed that the temperature of the polarizing plate on the incident side of the liquid crystal panel 105 is 70.degree. C. higher than that of the ambient air about the liquid crystal projector. Similarly, liquid cooler 115 is 30.degree. C. higher than that of the ambient air, because the black-matrix 109 of the liquid crystal panel 105, the color filter 110 and the polarizing plate 114 located on the emanating side of the liquid crystal panel 105 absorb light.
This increase in temperature of the device causes a high rate of degradation of physical and electrical characteristics of the liquid crystal layer 108, the polarizing plates 113, 114 and the color filter 110. To solve this problem, the quantity of light that enters into the liquid crystal layer 114 may be limited. But in such a case, the image that is projected by the projector is darkened and therefore, the quality of the projected image may be poor.
Further, in such a conventional liquid crystal projector, since the color liquid crystal panel 105 has a structure in which the liquid crystal layer 108 and the color filter 110 are combined as a unit, a defective liquid crystal layer 108 (resulting from a manufacturing process defect, for example) makes the whole color liquid crystal panel 105 useless even if the color filter 110 is in good condition, and vice versa. Therefore, the manufacturing cost of the liquid crystal panel 105 in which the liquid crystal layer 108 and the color filter 110 are combined as a unit is higher than that of a liquid crystal panel in which the liquid crystal layer and the color filter are not combined. Accordingly, in view of the relatively high rejection rate in the manufacturing process and the resulting high manufacturing cost, it is not desirable to use the structure of the liquid crystal panel 105 as shown in FIG. 17.